Observation of the Berezinskii-Kosterlitz-Thouless Phase Transition in an Ultracold Fermi Gas

TL;DR

This paper experimentally observes the Berezinskii-Kosterlitz-Thouless phase transition in a two-dimensional ultracold Fermi gas, revealing universal superfluid behavior across the BEC-BCS crossover with implications for quantum many-body physics.

Contribution

First experimental demonstration of BKT transition in a 2D ultracold Fermi gas across the BEC-BCS crossover, showing universality and fermionic effects.

Findings

Superfluid phase with algebraic correlations observed

Transition exponent remains constant across interaction strengths

Data aligns with quantum Monte Carlo for Bose gases in the bosonic regime

Abstract

We experimentally investigate the first-order correlation function of a trapped Fermi gas in the two-dimensional BEC-BCS crossover. We observe a transition to a low-temperature superfluid phase with algebraically decaying correlations. We show that the spatial coherence of the entire trapped system can be characterized by a single temperature-dependent exponent. We find the exponent at the transition to be constant over a wide range of interaction strengths across the crossover. This suggests that the phase transitions in both the bosonic regime and the strongly interacting crossover regime are of Berezinskii-Kosterlitz-Thouless-type and lie within the same universality class. On the bosonic side of the crossover, our data are well-described by Quantum Monte Carlo calculations for a Bose gas. In contrast, in the strongly interacting regime, we observe a superfluid phase which is significantly influenced by the fermionic nature of the constituent particles.